To make the question mandatory you can click on Settings and you will find a toggle button to turn ON the validation.
WHY? When you add questions to a survey, by default, required option is turned off. When required option is not enabled, respondents can continue with the survey without selecting answers. If respondents go through all the pages in the survey without selecting answers, the response is still considered as complete. You can enable required option to make a question required so that respondents can continue with the survey only after responding to the questions.

Once you put on the validation, you can set a character limit within which you want the text to be restricted.

License

This feature/tools described here are available with the following license(s) :